1. Field of the Invention
This invention relates to a process for the selective production of para-xylene by catalytic methylation of toluene in the presence of a particular crystalline aluminosilicate catalyst.
2. Description of the Prior Art
Alkylation of aromatic hydrocarbons utilizing crystalline aluminosilicate catalysts has heretofore been described. U.S. Pat. No. 2,904,607 to Mattox refers to alkylation of aromatic hydrocarbons with an olefin in the presence of a crystalline metallic aluminosilicate having uniform pore openings of about 6 to 15 Angstrom units. U.S. Pat. No. 3,251,897 to Wise describes alkylation of aromatic hydrocarbons in the presence of X- or Y-type crystalline aluminosilicate zeolites, specifically such type zeolites wherein the cation is rare earth and/or hydrogen. U.S. Pat. No. 3,751,504 to Keown et al. and U.S. Pat. No. 3,751,506 to Burress describe vapor phase alkylation of aromatic hydrocarbons with olefins, e.g., benzene with ethylene, in the presence of a ZSM-5 type zeolite catalyst.
Crystalline aluminosilicate zeolites, modified by reaction with an organic substituted silane, have been described in U.S. Pat. No. 3,682,996 to Kerr and in U.S. Pat. No. 3,698,157 to Allen et al. The former of these patents describes, as novel compositions of matter, crystalline aluminosilicate esters made by reacting a crystalline aluminosilicate having an available hydrogen atom with an organic silane having a SiH group. The resulting compositions were disclosed as being catalysts useful for hydrocarbon processes, particularly hydrocracking. In the latter of the above patents, the use of ZSM-5 type crystalline aluminosilicate zeolites modified by treatment with an organic-radical substituted silane are described, together with the use of such modified zeolites in chromatographic separation of the compounds contained in a C.sub.8 aromatic feed stock.
While the above-noted prior art is considered of interest in connection with the subject matter of the present invention, the methylation process described herein utilizing the specified controlled crystal size ZSM-5 type (including zeolite ZSM-11), or ZSM-21 crystalline aluminosilicate catalyst to achieve selective production of para-xylene has not, insofar as is known, been heretofore disclosed.
Of the xylene isomers, i.e., ortho-, meta- and para-xylene, the latter is of particular value being useful in the manufacture of terephthalic acid which is an intermediate in the manufacture of synthetic fibers such as "Dacron." Mixtures of xylene isomers either alone or in further admixture with ethylbenzene, generally containing a concentration of about 24 weight percent para-xylene in the equilibrium mixture, have been previously separated by expensive superfraction and multi-stage refrigeration steps. Such process, as will be realized, has involved high operation costs and has a limited yield.